In the production of particle boards, fiber boards or the like, belt-type prepresses are universally used for compacting the particle carpet into preshaped sections downstream of the continuously operating spreading station. This is necessary because the particle mat, which in the majority of cases is applied in three layers; the central layer comprising coarse particles and the two cover layers comprising fine particles, does not have sufficient cohesion to withstand transportation as far as the hot press, in particular the multi-daylight hot press, without sustaining damage. Deep layer particle mats would furthermore also require large clearances of the press openings in the hot presses. The consolidation of the particle mat or its surfaces furthermore permits more rapid closure of the hot presses since less account has to be taken of particles and dust blowing away due to the air to be displaced.
In practice however, difficulties are constantly being encountered in precompaction such as adhesion of the particles to the precompaction belt. A slow throughput is furthermore necessary in order to avoid dust particles being blown out of the upper cover layer. When a precompaction belt made of plastics is used, the air builds up in the run-in wedge during the compaction of the particle carpet and, when the dust fraction is high, leads to striation of the upper cover layer. This striation leads to a reduction in quality, i.e. minimization of surface quality during coating of the particle board right up to minimization of strength on account of crack formation in the upper cover layers.
The use of coarsely woven monofilament and multifilament belts as the precompaction belt is also known, and these have good air permeability on account of their coarse fabric structure. However, these have only a limited life of four to six weeks. In addition to the costs for the new belt, lengthy fitting times and an interruption in production of several hours are always the result.